The technologies of closed-loop pre-coding have been introduced to the Long Term Evolution (LTE) Release 8 (Rel-8) system to improve the spectrum frequency. The same set of pre-coding matrixes, which is referred to as a codebook, shall be stored in both a base station and a UE as required for the technologies of closed-loop pre-coding. The UE selects a pre-coding matrix from the codebook under some criterion after estimating channel information according to a cell common pilot, where the selected criterion can be the maximized amount of exchanged information, the maximized output signal to interference plus noise ratio, etc.; the UE feeds the index of the selected pre-coding matrix in the codebook back to the base station on an uplink channel, where the index is referred to as a Pre-coding Matrix Indicator (PMI); and the base station can determine the pre-coding matrix to be applied to the UE, according to the received index. The pre-coding matrix reported by the UE can be regarded as a quantized value of channel state information.
In the existing cellular system, an array of antennas in the base station is typically arranged horizontally (i.e., a linear array) as illustrated in FIG. 1 and FIG. 2. A beam of a transmitter in the base station can only be adjusted in the horizontal direction, and there is a fixed downward inclination angle of each UE in the vertical direction, so the various beam-forming/pre-coding technologies are applicable to channel information in the horizontal direction. In fact, a radio signal is propagated in three dimensions in a space, and the performance of the system cannot be optimized with a fixed downward inclination angle, so beam adjustment in the vertical direction is of great significance to an improvement in performance of the system. With the development of the antenna technologies, an array of active antennas in which respective array elements are controllable separately has emerged in the industry, for example, in a two-dimension arrangement of dual polarized antennas as illustrated in FIG. 3, and a two-dimension arrangement of liner array antennas as illustrated in FIG. 4. With such two-dimension antenna array (a planar array), it is possible to adjust a beam dynamically in the vertical direction. The base station also needs to perform beam-forming/pre-coding in three dimensions using channel state information reported by the UE. The LTE Rel-13 extends the codebook applied to a linear array to obtain a codebook of a planar array, so that the UE can feedback channel state information.
In the 3GPP LTE/LTE-A and IEEE 802.16 series of standards, various codebooks are designed for different numbers of antennas, antenna modalities, application scenarios, etc., but due to the limiting of the codebook structure, there is such limited precision of channel state information which can be provided in these codebook-based feedback schemes that the channel state information may not be applicable to a transmission scheme using a sophisticated preprocessing algorithm, e.g., Multi-User Multi-Input Multi-Output (MU-MIMO) transmission, at the base station side.